The invention relates to a braking device for use in a roller blind of the type having a spring-motor to wind up a screen on a tube and, more particularly, to a novel device for braking and releasing the rotation of the tube irrespective of the rotary speed of the tube.
A roller blind has both brackets, a fixed shaft secured to one of the both brackets, a tube having an end fitted on the fixed shaft with the intervention of a braking device and the other end rotatably supported by the other bracket, a screen wound on the tube, and a spring-motor interposed between the tube and the fixed shaft for winding the screen on the tube. The known braking device comprises a centrifugal clutch having detents, pins or balls as a centrifugal member and depends on the rotary speed of the tube. The centrifugal clutch never actuates the braking device unless the tube rotates to roll up the screen at such low speeds that a centrifugal force is too small to prevent the centrifugal members from engaging. On the contrary, it never releases the device unless the tube rotates at such high speeds that a centrifugal force is large enough to prevent the centrifugal members from engaging.
This leads to a disadvantage that the known braking device requires a little knack to be operated. The screen can not stop at a position below the uppermost position without receiving a somewhat delicate finger-resistance to reduce the winding speed and actuate the braking device, when it is released to be rolled up by the spring-motor after being pulled below the aforementioned position. The screen can not be wound up from any position below the uppermost position without being released immediately after it is rapidly pulled down to release the braking device. The trouble is that the braking device is not easy to be disengaged if it is easy to be engaged, and vice versa.
The invention provides a novel braking device having no centrifugal clutch, in order to avoid the disadvantages as caused by a centrifugal force. In accordance with the invention, for use with a roller blind of the type having both brackets, a fixed shaft non-rotatably supported by one of the both brackets, a tube having an end fitted on the fixed shaft and the other end rotatably supported by the other bracket, a screen wound on the tube, and a spring-motor interposed between the tube and the fixed shaft. The device comprises a brake drum non-rotatably mounted on the fixed shaft, a coil spring brake mounted on the brake drum, a cam mechanism interposed between the coil spring brake and the tube so as to loosen the coil spring brake when the screen is drawn and when it is released after drawn from any position except the uppermost position and tighten the coil spring brake on the brake drum when the screen is released after drawn from the uppermost position. The cam mechanism consists of a cam sleeve mounted on the brake drum and formed with a cam groove and a cam casing integrally mounted in the tube and provided with a cam pin for engagement with the cam groove. The cam groove is generally Y-shaped and centrally provided with a V-shaped land to form twin top inlets, a central inlet and a bottom inlet. The coil spring brake has one end thereof secured to the cam sleeve and the other end disposed under the bottom inlet of the Y-shaped groove. Whenever the screen is pulled down to a position from the uppermost position, the cam pin moves from the bottom inlet to one of the twin top inlets to push the one end of the coil spring brake with the intervention of the cam sleeve in the direction in which the coil spring brake loosens, resulting in that the tube rotates to pay out the screen. When the screen is released at the aforementioned position, the cam pin moves from the top inlet to the central inlet to push the one end of the coil spring brake in the direction in which the coil spring brake tightens on the brake drum with the result that the screen stops at the position. When the screen is released after drawn from the position, the cam pin moves from the central inlet to the bottom inlet through the other top inlet to push the other end of the coil spring brake in the direction in which the coil spring brake loosens, so that the screen is raised to the uppermost position by the spring-motor. If the Y-shaped groove is modified to have a common V-shaped inlet and an additional land replaced for the twin top inlets. The cam pin turns about the additional land and returns to the central inlet when the screen is released after drawn from a position by a length beyond a preselected distance, resulting in that the screen is gradually payed out from a position to another position. However, the cam pin moves from the central inlet to the bottom inlet when the screen is released after drawn by a length within the preselected distance.
It is a main object of the invention to provide a braking device that is easily operated irrespective of the winding speed of the tube.
It is a further object of the invention to provide a braking device by which the screen is stopped at any position with ease.
It is a still further object of the invention to provide a braking device by which the screen is gradually payed out from a position to another position, if desired.